My husband is definitely my best friend. We started off as friends and it quickly grew. I know I can tell him almost anything, and at the end of the day, he's the one I'm most comfortable with. I can't really say I have a best girl friend anymore, as I no longer talk to the one from high school who was my maid of honor (no real reason, just life on separate sides of the country). I have a lot of close girl friends, but they're all online. I don't have anyone nearby that I could just get together with. Yeah, I don't get out much.
But it works for me.
I've always had the idea that you *can* have multiple best friends. Different people fit into your lives in different ways. Growing up, I always had at least two best friends who usually never interacted. So having your spouse as a best friend definitely does not mean you can't have a best girl friend, too.